Easier positioning accuracy management with Network RTK: always cm-level accuracy (half-inch accuracy) with real-time corrections

Even a position shift of only a few centimeters can have serious consequences on construction sites and in civil engineering surveys. For example, in tasks such as setting out foundations, driving piles for roadworks, or as-built control, positioning errors determine quality and safety. Therefore, expectations for high-precision positioning technologies using GNSS are rising, and among them Network RTK is attracting attention as a method that can realize centimeter-level positioning in real time. This article explains the GNSS positioning mechanism and error sources, the differences and advantages of RTK and Network RTK, and the importance and methods of managing positioning accuracy in real time. It also touches on ensuring traceability of survey records through digitalization and the benefits of instant data sharing from the field, and concludes with a look at latest trends such as LRTK (high-precision smartphone positioning) and the integration of RTK with AR.

How GNSS positioning works and error sources

First, let’s cover the basics of positioning using GNSS (Global Navigation Satellite Systems). GNSS positioning receives radio signals transmitted from multiple satellites and calculates the distance to each satellite from the signal propagation time, thereby determining the current position. Satellite constellations such as GPS, GLONASS, Galileo, and QZSS (Michibiki) enable position information anywhere on Earth. However, in the case of standalone positioning, accuracy remains on the order of several meters due to various error sources.

Major GNSS error sources include the following:

• Satellite-side errors: Errors caused by slight deviations in satellite orbit or clocks. If the orbit information (ephemeris) or timing information sent from the satellite is not perfectly accurate, range measurements will include errors.

• Atmospheric delays: Errors caused by delays when radio waves pass through the ionosphere and troposphere. The ionosphere’s electron density changes due to solar activity and refracts radio waves. Tropospheric effects such as water vapor and pressure also change propagation speed. These cause shifts in signal travel time and introduce range errors.

• Multipath (reflections): This occurs when satellite signals reflect off buildings or the ground surface before reaching the receiver. If the receiver captures reflected waves that traveled a longer path than the direct signal, it will misinterpret the distance as longer and introduce positioning errors. This error source is especially problematic in urban and mountainous areas.

• Receiver noise and positioning geometry: Measurement noise within the receiver also has a small impact. Accuracy also varies depending on the geometry of the satellites in view. Poor satellite geometry increases uncertainty in position computation (worsened DOP values), degrading accuracy.

Due to these factors, ordinary GNSS standalone positioning can sometimes result in errors around 10 m (32.8 ft). For applications such as car navigation or smartphone map apps, this accuracy is sufficient, but meter-level errors are unacceptable in construction surveying. Therefore, relative positioning techniques that use multiple receivers to cancel errors become important.

Relative positioning means observing the same satellites simultaneously with two or more GNSS receivers and comparing observation data between the point to be positioned and a known point to cancel errors. Satellite signals received at the same instant contain common error components (from satellites and the atmosphere), and taking the difference between two points allows you to cancel common errors. GNSS also offers very precise range measurement using the carrier phase of the radio wave (carrier-phase positioning), and combining this method makes it possible to achieve sub-centimeter accuracy. RTK realizes high precision precisely by combining relative positioning and carrier-phase measurements.

Differences and advantages of RTK positioning and Network RTK

RTK positioning (Real Time Kinematic) is a method that dramatically improves GNSS accuracy through real-time relative positioning. Typically, a receiver called a reference station (base station) is installed at a point with known coordinates, and the reference station and the survey receiver acting as a rover (mobile unit) observe multiple satellites simultaneously. The reference station calculates error amounts in the direction of each satellite from its known position and the received satellite signals, and continuously transmits correction information to the rover via radio or the Internet. The rover applies those corrections to its own satellite measurements and computes its position in real time with centimeter-level accuracy.

With RTK, many satellite orbit/clock errors and atmospheric delays that cannot be removed by standalone positioning are canceled by differencing, so accuracy improves dramatically. A major advantage of RTK is that it can immediately provide horizontal positioning accuracy of about 1–2 cm (0.4–0.8 in), comparable to traditional optical distance measurement or leveling-based surveys. This makes it possible to perform pile positioning and as-built measurements quickly and with high precision.

However, traditional RTK had one weakness: to obtain high accuracy, the reference station must be installed near the survey site. As the distance between the reference station and the rover (the baseline length) increases, error components that cannot be shared between them (particularly ionospheric and tropospheric effects) increase and corrections become less effective, so accuracy gradually degrades. Generally, for single RTK operations, it is desirable to keep the distance between the reference station and the rover within 10 km, and if farther, it may take longer to obtain a fixed solution (see below) or accuracy may degrade by several centimeters or more. Therefore, surveyors had to set up their own reference stations near the work area and transmit correction data by radio, which required time and effort.

To solve this problem, the method called Network RTK was developed. Network RTK uses a pre-established network of multiple reference stations spread over a wide area (such as continuously operating reference station networks), allowing users to obtain high-precision correction information without installing a reference station on site. A representative technology is the VRS (Virtual Reference Station) method. In the VRS method, the user’s (rover’s) approximate position is used by a server to integrate observation data from several nearby reference stations and create a virtual reference station near the user. The server then generates correction information as if observations had been made at that virtual reference station and delivers it to the user via a communication line (mainly via mobile Internet using the Ntrip protocol). This gives the user the equivalent of having a reference station right next to them, enabling RTK positioning without worrying about baseline length–related accuracy degradation.

Network RTK (including VRS) has many advantages. First, there is no need to set up a reference station, greatly streamlining survey preparation. You only need to bring one receiver to the site, freeing you from selecting and installing reference points and managing equipment. Second, it provides uniform high accuracy over a wide area. Because the virtual reference point is always assumed to be near the measurement location, stable centimeter-level positioning is possible anywhere in the service area in the country. For example, in Japan, the Geospatial Information Authority operates a real-time correction service using about 1,300 continuously operating reference stations (GNSS observation network), and by using this network RTK, you can obtain high-precision coordinates in the global geodetic system instantly without placing your own reference station on site. Private companies also offer network RTK services using mobile networks, deploying many reference stations across the country to expand service areas. As a result, surveyors can easily obtain centimeter-level positioning across a wide range—from mountainous areas to urban environments—so long as communication conditions are met.

Why real-time management of positioning accuracy is necessary (re-surveying and quality assurance)

Although Network RTK has made centimeter accuracy easy to obtain, real-time management of positioning accuracy in the field remains an important issue. Even with high-precision survey equipment, unexpected errors or mistakes can occur if accuracy is not properly monitored. Discovering errors only after returning to the office to inspect the data can be too late, forcing you to go back to the site for re-surveying, which leads to time and cost losses, project delays, and duplicated work.

In addition, the construction industry today requires strict control and records of survey data accuracy from the perspectives of quality assurance and compliance. For example, as-built control requires retaining data that can demonstrate survey accuracy for quantity inspections, and public works have inspection standards for survey results, making accuracy management indispensable as part of quality assurance. Managing positioning accuracy in real time not only prevents mistakes at the site but also supports the reliability and future verifiability of the data.

The underlying shift is from a “measure once and done” mindset to verifying data quality on the spot. Traditionally, surveyors using total stations performed cross-checks and known-point comparisons on site to eliminate errors, and the same attitude of questioning and verifying results in real time is essential for GNSS surveying as well. In Network RTK, automatic high-precision coordinates may give a false sense of security, but communication issues or changes in satellite reception can sometimes degrade accuracy. Detecting and responding to such issues immediately is key to ensuring quality and avoiding rework.

How to check errors in real time, the meaning of a fixed solution, and precautions

So how should you actually confirm RTK positioning accuracy on site? GNSS receivers and controller terminals display the current positioning status and accuracy indicators in real time. To decide “Can I trust this data?” check the following points:

• Check the solution type (solution status): The basic check is the type of GNSS solution. Many RTK-capable devices display whether the current solution is “FIX” (fixed) or “FLOAT” (float), or if it is just standalone (Single) or DGPS-augmented, etc. A fixed solution (Fix) means that the integer ambiguities of the GNSS carrier-phase have been resolved, and only when Fix is obtained is centimeter-level accuracy guaranteed. Conversely, a float solution (Float) indicates integer ambiguities have not yet been resolved, and accuracy remains on the order of several tens of centimeters to about 1 m (3.3 ft). Therefore, when recording a survey point, always confirm that the solution is FIX. If the display shows “FLOAT,” “DGNSS,” “SINGLE,” or anything other than Fix, high precision is not being achieved and caution is required.

• Monitor whether the FIX solution is maintained: Always watch whether FIX is maintained throughout measurements. For example, entering a building’s shadow may immediately drop the solution to Float. If you continue measuring without noticing, you may record values with large errors. If it temporarily falls to Float, promptly take measures such as moving away from obstructions, raising the antenna height, or reacquiring reference station data, and resume point observations only after Fix is restored. It is also important to wait until a stable FIX solution is obtained. Since satellite configuration and ionospheric conditions change by season and time of day, waiting a bit under poor conditions can sometimes improve accuracy.

• Check positioning accuracy indicators (DOP values and estimated errors): Receivers also display position accuracy indicators in real time. A representative indicator is the DOP value (Dilution of Precision), which reflects accuracy degradation due to satellite geometry. In particular, a high HDOP value (for horizontal precision)—for example, exceeding about 2.0—indicates satellites are clustered in one direction and positional uncertainty is increasing. Even with a Fix, poor HDOP can lead to larger horizontal errors, so move to a location with better sky visibility or wait for improved satellite geometry. Some models also numerically display real-time estimated errors (e.g., horizontal ±○ cm (±○ in), vertical ±○ cm (±○ in)). Use these as references to continuously confirm on site that your positioning accuracy meets specifications.

By monitoring errors in real time as described above, you can detect and respond to problems immediately. However, be careful not to overtrust a fixed solution. Even with a Fix indicator displayed, there is a nonzero risk of a false fix in environments with severe multipath or when the reference station is too far away. For critical observation points, double-checking—such as performing repeated observations at different times and comparing results, or testing on known points to verify accuracy—is effective. Do not assume a fixed solution equals absolute safety; on-site judgment rather than blind reliance on the device is required for accuracy control.

Digital forms, ensuring traceability, and instant sharing from the field

As RTK and Network RTK have spread, survey data handling has rapidly moved toward digitalization. Observed coordinates and accuracy information are recorded and stored electronically, marking a major change from the traditional practice of writing notes in paper field books. Dedicated controllers and tablet apps can generate field measurement results directly as digital forms, reducing transcription errors and speeding up record-keeping. Digitized positioning data can be automatically aggregated into in-house systems or easily shared with project stakeholders via the cloud.

A major advantage of digital forms is superior traceability of survey results. History information—who measured what, where, and how—can be linked to the data so later verification and tracing are possible. For RTK surveys, metadata such as observation date/time, number of satellites used, DOP values, and solution status (Fix/Float) can be automatically recorded. This allows you to reproduce survey conditions later and analyze causes if questions arise like “Why did this point’s position deviate?” For quality management, electronic data also facilitates tamper prevention and backups, making storage more reliable than paper. It also eases compliance with public works requirements for electronic deliverables and the submission of evidence during inspections.

Moreover, real-time positioning with Network RTK enables instant sharing from the field. Uploading measured coordinate data to the cloud on the spot, or sending it over mobile communications to an office server, allows designers and supervisors in remote locations to share data immediately. For example, if a sudden change occurs on site, a staff member can measure and upload the latest coordinate data to the cloud so headquarters design staff can immediately grasp the situation and consider countermeasures. Instant sharing of survey data bridges the information gap between the field and the office, speeding decision-making and preventing rework.

There are other benefits to instant field sharing. Backing up data immediately after measurement protects it if a tablet is dropped and broken. If survey data accumulates on a cloud-sharing platform, all stakeholders can view the same up-to-date information, preventing duplicate measurements and reducing communication losses. The combination of real-time positioning and digital connectivity dramatically improves site productivity and quality management.

Potential developments: simple surveying with LRTK and expansion to RTK-AR integration

Finally, let us touch on future developments enabled by Network RTK. Recently, simple RTK surveying systems combining smartphones or tablets with GNSS receivers have begun to emerge. Technologies referred to as LRTK use ultra-compact high-precision GNSS antennas attachable to smartphones and receive Network RTK correction information via the smartphone to achieve centimeter-level positioning on a palm-sized device. This opens up an era where anyone with a smartphone can carry out surveying without specialized equipment. Small contractors without expensive gear can adopt high-precision surveying easily, streamlining routine surveys and as-built checks.

The integration of RTK with AR (augmented reality) technology also opens new possibilities. AR overlays virtual design data on live camera views on smartphones or tablets, serving as an intuitive field support tool. Combining AR with high-precision RTK positioning makes it possible to project digital drawing lines or structural models precisely at the correct locations on site. For example, tasks that previously required comparing drawings with surveying equipment for pile driving or batter board layout could be guided directly by AR displays on a tablet, enabling users to place piles at exact positions by following on-screen guides. AR can also display the positions of underground utilities to aid excavation or project a 3D finished model on site to check appearance; high-precision positioning × AR will increasingly drive field digital transformation (DX).

Network RTK underpins these next-generation solutions. Because it provides accurate real-time positions, smartphone surveying and AR-assisted construction become feasible. As satellite positioning technology advances further—such as expanded use of the L5 band and wider adoption of Japanese centimeter-level augmentation services (Michibiki CLAS, PPP-RTK)—even easier and more stable centimeter-level positioning will be realized. The benefits will extend beyond surveyors and engineers to construction machine operators, craftsmen, and potentially general users. Starting from real-time accuracy management using Network RTK, construction and surveying are clearly moving to a new stage. Amid this democratization of high-precision positioning, engineers should aim for smart site operations that maximize the advantages of these new methods while balancing quality and efficiency.